Simplify the following expression: $r = \dfrac{5a}{3} + \dfrac{8a}{7}$
Explanation: In order to add expressions, they must have a common denominator. The smallest common denominator is the least common multiple of $3$ and $7$ $\lcm(3, 7) = 21$ $ r = \dfrac{7}{7} \cdot \dfrac{5a}{3} + \dfrac{3}{3} \cdot \dfrac{8a}{7} $ $r = \dfrac{35a}{21} + \dfrac{24a}{21}$ $r = \dfrac{35a +24a}{21}$ $r = \dfrac{59a}{21}$